Product for a welded construction made of AlMgMn alloy having improved mechanical strength

ABSTRACT

Rolled or extruded products for welded constructions are made of an aluminum-magnesium-manganese type aluminum alloy, consisting essentially of, by weight: 
     3.0&lt;Mg&lt;5.0 
     0.5&lt;Mn&lt;1.0 
     Fe&lt;0.25 
     Si&lt;0.25 
     Zn&lt;0.40 
     Cr&lt;0.25 
     Cu&lt;0.20 
     Ti&lt;0.20 
     Zr&lt;0.20 
     The product has a volumetric fraction of Mn containing dispersoids grater than 1.2%.

FIELD OF THE INVENTION

The invention relates to the sphere of rolled or extruded products such as sheets, profiles, wires or tubes made of AlMgMn-type aluminium alloy containing more than 3% by weight of Mg, intended for welded constructions having a high yield stress, good resistance to fatigue and good toughness for structural applications such as ships, industrial vehicles or welded bicycle frames.

DESCRIPTION OF THE RELATED ART

The optimum dimensioning of welded structures made of aluminium alloy leads to the use of 5,000 series AlMg alloys according to the Aluminium Association nomenclature, in the cold-worked temper (temper H1 according to the standard NF-EN-515) or partially softened temper (temper H2), or stabilized temper (temper H3), while maintaining high resistance to corrosion (temper H116) rather than the annealed temper (temper O). However, the improvement in the mechanical characteristics relative to the temper O does not usually remain after welding, and certifying and monitoring organizations generally recommend that only the characteristics in temper O be taken into consideration for welded structures. The resistance to fatigue and the fissure propagation rate should also be taken into consideration for dimensioning.

In this sphere, research has concentrated mainly on the implementation of the welding operation itself. There have also been attempts to improve the corrosion resistance of the article by appropriate thermomechanical treatments.

Japanese patent application JP 06-212373 proposes the use of an alloy containing 1.0 to 2.0% of Mn, 3.0 to 6.0% of Mg and less than 0.15% of iron to minimize the reduction in the mechanical strength due to welding. However, the use of an alloy having such a high manganese content leads to a reduction in the resistance to fatigue and in the toughness.

SUMMARY OF THE INVENTION

The object of the invention is significantly to improve the mechanical strength and fatigue resistance of welded structures made of AlMgMn alloy, under predetermined welding conditions, without unfavourable consequences for other parameters such as toughness, corrosion resistance and cutting deformation, due to internal stresses.

The invention relates to products for welded constructions made of AlMgMn aluminium alloy composed of (% by weight):

3.0<Mg<5.0

0.5<Mn<1.0

Fe<0.25

Si<0.25

Zn<0.40

optionally one or more of the elements Cr, Cu, Ti, Zr such that:

Cr<0.25

Cu<0.20

Ti<0.20

Zr<0.20

other elements <0.05 each and <0.15 in total, wherein Mn+2Zn>0.75.

DETAILED DESCRIPTION OF THE INVENTION

Contrary to earlier research which concentrated on the welding process and the thermomechanical treatments, the inventors have found a particular, range of composition for minor alloying elements, in particular iron, manganese and zinc, leading to an interesting set of properties combining static mechanical characteristics, toughness, resistance to fatigue, resistance to corrosion and cutting deformation, this set of properties being particularly well adapted to the use of these alloys for naval construction, utility vehicles or the welded frames of bicycles.

This set of properties is obtained by combining a low iron content, <0.25%, preferably <0.20%, and even 0.15%, and a manganese and zinc content such that Mn+2Zn>0.75%, preferably >0.8%. The Mn content should be >0.5%, preferably >0.8%, to have adequate mechanical characteristics, but should not exceed 1% if a deterioration in toughness and fatigue resistance are to be avoided. The addition of zinc combined with manganese has been found to have a beneficial effect on the mechanical characteristics of welded sheets and joints. However, it is better not to exceed 0.4% because problems can then be encountered in welding.

The magnesium is preferably kept >4.3%, because it has a favourable effect on the yield stress and fatigue resistance, but beyond 5% the corrosion resistance is less good. The addition of Cu and Cr are also favourable to the yield stress, but Cr is preferably kept <0.15% to maintain good resistance to fatigue.

The mechanical strength of the sheets depends both on the magnesium content in solid solution and on the manganese dispersoids. It has been found that the volumetric fraction of these dispersoids, which is linked to the iron and manganese contents, should preferably be kept above 1.2%. This volumetric fraction is calculated from the average of the surface fractions measured on polished cuts produced in three directions (length, width and thickness) by scanning electron microscopy and image analysis.

The products according to the invention can be rolled or extruded products such as hot- or cold-rolled sheets, wires, profiles or extruded and optionally drawn tubes.

The sheets according to the invention, which are assembled by butt welding by a MIG or TIG process and with a bevel of the order of 45° over about ⅔ of the thickness have, in the welded region, a yield stress R_(0.2) which can be at least 25 MPa higher than that of a conventional alloy having the same magnesium content, that is a gain of about 20%.

The width of the thermally affected region is reduced by about one third relative to a conventional 5083 alloy, and the hardness of the welded joint increases from about 75 Hv to more than 80 Hv. The welded joints also have a tensile strength exceeding the minimum imposed by organizations monitoring unwelded cold-worked crude sheets.

The sheets according to the invention have fatigue resistance, measured by plane bending with a stress ratio wherein R=0.1 on samples taken in the cross-longitudinal direction, higher than:

10⁵ cycles with a maximum stress >280 MPa

10⁶ cycles with a maximum stress >220 MPa

10⁷ cycles with a maximum stress >200 MPa.

The fissure propagation rate ΔK, measured when R=0.1, is >22 Mpa{square root over (m)} when da/dN=5×10⁻⁴ mm/cycle and >26 Mpa{square root over (m)} when da/dN=10⁻³ mm/cycle.

The sheets according to the invention usually have a thickness greater than 1.5 mm. With thicknesses greater than 2.5 mm they can be obtained directly by hot rolling, without the need for subsequent cold rolling and, furthermore, these hot-rolled sheets are less distorted on cutting than cold-rolled sheets.

The products according to the invention have corrosion resistance which is as good as that of normal alloys having the same magnesium content, for example 5083 of common composition, widely used in naval construction.

EXAMPLE

Thirteen samples of sheets were prepared by conventional semicontinuous casting in the form of plates, were heated for 20 h at a temperature >500° C. and were then hot-rolled to the final thickness of 6 mm. The reference 0 corresponds to a conventional 5083 composition and reference 1 to a composition slightly outside the invention. The 11 others (references 2 to 12) have a composition according to the invention.

The compositions were as follows (% by weight):

Ref Mg Cu Mn Fe Cr Zn Ti Zr 0 4.40 <0.01 0.50 0.27 0.09 0.01 0.01 1 4.68 <0.01 0.72 0.12 0.05 <0.01 0.01 2 4.56 <0.01 0.83 0.12 0.13 0.01 0.01 3 4.60 <0.01 0.85 0.17 0.10 0.16 0.01 4 4.62 <0.01 0.96 0.10 0.05 0.02 0.01 5 4.80 0.09 0.80 0.11 0.03 0.02 0.01 6 4.72 <0.01 0.87 0.13 0.03 0.02 0.01 0.11 7 4.88 0.05 0.78 0.16 0.02 0.01 0.09 8 4.92 0.06 0.94 0.08 0.02 0.19 0.01 9 4.69 <0.01 0.72 0.07 0.02 0.10 0.01 10 4.71 <0.01 0.82 0.06 0.02 <0.01 0.01 11 4.73 <0.01 0.95 0.17 0.03 <0.01 0.01 12 4.70 <0.01 0.92 0.22 0.03 0.01

The samples all have, after rolling, a yield stress R_(0.2)>220 Mpa in the L direction.

The mechanical strength of the joints welded from these sheets was measured under the following conditions: continuous automatic MIG butt welding with a symmetrical bevel having an inclination of 45° to the vertical over a thickness of 4 mm and filler wire of 5183 alloy.

The mechanical characteristics (tensile strength R_(m), yield stress R_(0.2)) were obtained by pulling over samples standardized by the Norwegian monitoring organization DNV for naval construction having a length of 140 mm and a width of 35 mm, the weld bead with a width of 15 mm being in the centre and the length of the narrow portion of the sample being 27 mm, that is the sum of the width of the bead and twice the thickness (15+22 mm).

The volumetric fractions of manganese dispersoids was also measured.

The results are as follows (in MPa for resistances and % for fractions):

Ref. R_(m) R_(0.2) Fractions 0 285 131 0.62 1 292 144 1.2 2 302 150 1.4 3 300 146 1.6 4 310 158 1.7 5 309 149 1.4 6 305 155 1.5 7 315 166 1.3 8 318 164 1.9 9 310 153 1.5 10 312 150 1.5 11 315 153 1.6 12 315 151 1.5

It is found that the yield stress of samples welded according to the invention increases by between 15 and 35 MPa relative to the reference sample.

The resistance to fatigue of unwelded sheets subjected to plane bending wherein R=0.1 was also measured for references 0 to 5, while determining the maximum stress (in MPa) corresponding to 10⁶ and 10⁷ cycles respectively, as well as the fissure propagation rate ΔK measured when da/dn=5×10⁻⁴ mm/cycle (in Mpa{square root over (m)}).

The results were as follows:

Ref. 10⁶ cycles 10⁷ cycles ΔK 0 220 200 22 1 235 205 22 2 230 200 23 3 225 200 23 4 230 205 22 5 225 200 22

It is found that, despite the increase in the mechanical strength, the sheets according to the invention have resistance to fatigue which is at least as good as that of conventional 5083 sheets. 

What is claimed is:
 1. A welded construction comprising first and second sections made of AlMgMn aluminum alloy welded together to form a joint having a hardness >80 Hv, said alloy consisting essentially of (% by weight): 4.3<Mg<5.0 0.5<Mn<1.0 Fe<0.20 Si<0.25 Zn<0.40 Cr<0.25 Cu<0.20 Ti<0.20 Zr<0.20 wherein Mn+2Zn>0.75, each of said sections having been formed by casting, homogenizing, hot rolling and optionally cold rolling, wherein the welded joint, when prepared by continuous automatic MIG butt welding with a symmetrical bevel having an inclination of 45° to vertical over a thickness of 4 mm and with filler wire of 5183 alloy, determined by a tensile test on standardized DNV test pieces having a length of 140 mm and a width of 35 mm, a center weld bead with a width of 15 mm, and a length of a narrow portion which is the sum of bead width and twice section thickness, has a yield stress R_(m) greater than 300 MPa.
 2. Welded construction according to claim 1, wherein Mg>4.3%.
 3. Welded construction according to claim 1, wherein Mn>0.8%.
 4. Welded construction according to claim 1, wherein Fe<0.15%.
 5. Welded construction according to claim 1, wherein said first and second sections are in the form of sheets of thickness >2.5 mm which are only hot rolled.
 6. Welded construction according to claim 1, wherein the first and second sections have fatigue resistance, measured by plane bending wherein R=0.1 in the cross-longitudinal direction, higher than: 10⁵ cycles with a maximum stress >280 MPa; 10⁶ cycles with a maximum stress >220 MPa; and 10⁷ cycles with a maximum stress >200 MPa.
 7. Welded construction according to claim 1, wherein the first and second sections have a fissure propagation rate ΔK, measured when R=0.1, higher than: 22 Mpam when da/dn=5×10⁻⁴ mm/cycle; and 26 Mpam when da/dn=10⁻³ mm/cycle.
 8. Welded construction according to claim 1, wherein said alloy has a fraction of Mn-containing dispersoids greater than 1.2%.
 9. Welded construction according to claim 1, which forms part of a naval construction.
 10. Welded construction according to claim 1, which forms part of an industrial vehicle.
 11. Welded construction according to claim 1, wherein the first and second sections are welded tubes which form part of a bicycle.
 12. Welded construction according to claim 1, wherein Mn+2Zn>0.80.
 13. Product according to claim 1, wherein Zn is at least 0.01. 